A centrifugal pump is composed basically of a casing within which is rotatably mounted a driving or driven shaft that has an impeller attached at one end. Impellers used in a centrifugal pump are normally classified in one of four categories--a radial-type impeller, Francis-type impeller, mixed-flow-type impeller, and axial or propeller-type impeller. In general, the highest head is obtained from a radial-type impeller centrifugal pump and the lowest head is obtained from an axial-type impeller centrifugal pump. Intermediate heads are obtained from the Francis-type or Francis-vane impeller centrifugal pump and the mixed-flow-type impeller centrifugal pump with the Francis-vane impeller centrifugal pump producing more head than is normally obtained from the mixed-flow-type impeller centrifugal pump.
In general terms, the more head obtained from a centrifugal pump corresponds with a smaller capacity being processed through the pump. Also, axial-type impeller pumps are more efficient than radial-type impeller pumps and the efficiencies of the other two types of impeller pumps lie therebetween.
Radial-type impellers produce high head and low capacity at low efficiency and prior Francis-type impellers produce low head and high capacity at high efficiency. Examples of high speed centrifugal pumps in this general area using a Francis-type impeller are shown in U.S. Pat. Nos. 3,817,653; 3,935,833; 3,953,150; 3,981,626; 4,004,541 and 4,031,844. With the pump of the present invention, radial-type head can be obtained with Francis-type efficiency. That is, the pump of the present invention operates in the radial-type zone with Francis-type efficiency.